If you wish to use these buildings all year round you will need in insulate the sides, roof and ideally the floor. You would need to put fibre glass loft type insulation between the internal wall timbers and then cover this with plywood or plasterboard. I would recommend that you put 2" thick polystyrene in the roof space before covering it with plywood.
The type of glazed doors needs serious consideration as the more glass there is the more likely it is to be hot in summer or cold in winter as you say. Even with the building insulated you would need some form of cooling in the summer and some form of heating in the winter. This is because a single storey building does not have a loft to help retain heat in winter or help to keep it cool in summer.
Titan are now able to offer double glazed doors and windows to improve your comfort in your new Titan Garden Buildings Home Office.
Regards,
Robin
1st Choice Leisure Garden Buildings for timber sheds, summer houses, playhouses, heavy duty workshops and garages in timber, concrete, metal and plastic.
See
http://www.leisurebuildings.com/supp...=&threadid=229 and
http://www.leisurebuildings.com/crane/home-offices.html for more details of insulation options available for home garden offices